Fundamental characteristics required for photographic silver halide emulsions are high sensitivity, low fog, fine granularity and high development activity. Though silver halide includes silver fluoride, silver chloride, silver bromide and silver iodide, silver fluoride is not used for photographic emulsions because the silver fluoride has high water solubility. Accordingly, efforts to improve the fundamental characteristics of emulsions have been performed by combining silver chloride, silver bromide and silver iodide. Light absorption increases in the order of silver chloride, silver bromide and silver iodide, but development activity decreases in the above described order. Therefore, it is difficult for the light absorption to be compatible with the development activity. E. Klein and E. Moisar have disclosed mixed silver halide emulsions comprising silver halide cores coated with different silver halide layers (specifically, the grain is composed of a silver bromide nucleus, a first layer composed of silver iodobromide containing 1% by mol of silver iodide, and an external layer composed of silver bromide), by which light-sensitivity is enhanced without damaging development activity. (Japanese Patent Publication No. 13162/68)
Koitabashi et al have disclosed that photographically suitable characteristics, such as improvement of covering power, etc., are obtained when a thin shell having a thickness of 0.01 to 0.1 .mu.m is coated on a core grain having a comparatively low silver iodide content. (Japanese Patent Application (OPI) No. 154232/82) (The term "OPI" as used herein refers to a "published an unexamined Japanese patent application".)
These inventions are useful in case that the core part has a low silver iodide content and, consequently, the total silver iodide content is low. However, in order to obtain higher sensitivity and higher quality images, the silver iodide content of the emulsions must necessarily be increased.
It is well known that light absorption increases up to a limited amount of solid solution (about 45% by mol) with increases of the silver iodide content. Accordingly, it has been desired to develop emulsions containing cores having a high silver iodide content and having a high total silver iodide content. Silver iodobromide emulsions containing cores having a high silver iodide content have been disclosed in Japanese Patent Publication No. 21657/74 by Arai and Ueda. According to this patent publication, cores containing 25% by mol or 40% by mol of silver iodide are prepared and, thereafter, the shell is formed thereon by adding potassium bromide and an aqueous solution of silver nitrate.
X-ray diffraction of the resulting grains shows that the prepared emulsion does not have a complete core/ shell structure (Japanese Patent Publication No. 21657/74). As shown in the example at column 5, lines 40-45, it is very difficult to develop silver halide emulsions containing' cores having a high silver iodide content and having a high total silver iodide content, which have a distinct stratiform structure.